There are presently two general types of automatic, body-implantable pulse generators used in the treatment of cardiac disorders: pacemakers and defibrillators. These devices are designed to treat abnormal rhythms of the heart known generally as arrhythmias. They are miniature electrical pulse generators typically implanted in either the thorax or the abdominal cavity of the patient and are electrically coupled to the heart by insulated leads with one or more conductors therein for communicating electrical signals between the implanted device and cardiac tissue.
The sinus node is the heart's natural pacemaker. It generates electrical stimuli which are conducted through specialized nerve-like tissue (the atrio-ventricular node) to the lower chambers of the cardiac muscle, affecting muscular contraction of the chambers. During a so-called bradyarrhythmia, conduction of electrical impulses in the heart is partially or completely blocked, causing a delayed contraction or no contraction. An implantable pulse generator functions to maintain the natural rhythm of the heart by sending electrical pulses to the heart in place of a malfunctioning sinus node. The electrical pulses sent by the pulse generator stimulate the heart back into its natural rhythm.
An implantable cardioverter-defibrillator (ICD) functions to restore a patient's heart to a normal rhythm upon detection of cardiac tachyarrhythmia or cardiac fibrillation. In general, the magnitude of cardioverting or defibrillating pulses necessary to terminate an episode of tachyarrhythmia or fibrillation is greater than that of pacing pulses.
Generally speaking, implantable pulse generators can be characterized as being comprised of three primary components: a control circuit, an output circuit, and a power source. An implantable device's control circuit determines, among other things, the rate, synchronization, pulse width, and output voltage of heart stimulating pulses that are generated by the pulse generator. The control circuit may also perform diagnostic functions which are necessary to the safe operation of the generator. An implantable device's output circuit generates electrical stimulating pulses to be applied to the heart via one or more leads in response to signals from the control circuit.
Lithium batteries are generally regarded as acceptable power sources for implantable devices, due in part to their high energy density and low self discharge characteristics relative to other types of batteries. Power must be applied to both the control circuit and output circuit of an implantable device. Typically, the control circuit draws relatively little power from the battery, but must be continuously supplied in order to ensure the ongoing operation of the electronic circuitry typically associated therewith. The output circuit, on the other hand, requires a relatively larger amount of power but draws it mainly during peak demand periods.
In some cases, the power requirements of an implantable device's output circuit are higher than the battery can deliver. Thus, it is common in the prior art to accumulate and store the stimulating pulse energy in an output energy storage device at some point prior to the delivery of a stimulating pulse. (Most often, the output energy storage device comprises a capacitor, although it is contemplated that other types of energy storage devices could serve this purpose. For the sake of simplicity, however, the term "output capacitor" will be used herein to refer to the energy storage device in the output circuit.) When the control circuit indicates to the output circuit that a stimulating pulse is to be delivered, the output circuitry causes the energy stored in the output capacitor to be applied to cardiac tissue via the implanted leads. Prior to delivery of a subsequent stimulating pulse, the output capacitor must be recharged.
One perceived drawback of prior implantable pulse generators is that they often have to be replaced before their battery depletion levels have reached a maximum. When an implantable device's output capacitor is being recharged, there is a drop in battery voltage due to the charging current flowing through the battery impedance. Although this voltage drop may not be significant when the battery is fresh, over time the battery ages, causing its internal impedance increase or its peak output voltage to decrease, such that the voltage supplied to the control circuit may drop below a minimum allowable level. This temporary drop can cause the control circuit to malfunction. The implantable pulse generator must be removed and replaced before any such malfunctions occur, even though the battery may still have sufficient capacity to stimulate the heart. For this reason, most implantable device manufacturers specify battery end-of-life (EOL) criteria which are met prior to complete depletion of the battery.
In the context of cardiac pacemakers, one approach to the problem of battery voltage drop during output capacitor recharging has been to provide a priority switching circuit which provides a minimum voltage to the control circuit while the output capacitor is being recharged. Such an arrangement is disclosed, for example, in U.S. Pat. No. 4,599,523 to Pless et al. entitled "Power Priority System." The device disclosed in Pless a priority switching circuit which utilizes a hold-up capacitor of approximately 10- .mu.F which is connected in parallel with the control circuit and which is charged during the off-peak demand cycles of the charging/output circuit. In a peak demand cycle, the battery is disconnected from the control circuit and connected to the output circuit. During this time, the hold-up capacitor supplies the control circuit with a minimum operating voltage. The switching circuit of this device is designed to default to the control circuit, so that if the drain on the hold-up capacitor is too great during charging of the output capacitor, the battery is temporarily disconnected from the output circuit and reconnected to the control circuit, also recharging the hold-up capacitor. This insures that a minimum voltage is always supplied to the control circuit. This approach is feasible in the context of a cardiac pacemaker, which simply charges a capacitor to a few volts, imposing a high a high current drain on the battery for a period of 10-20 milliseconds, as the hold-up capacitor can be quite small.
In the context of an implantable defibrillator, which may charge an output capacitor bank to over 700 volts, over a period of up to 30 seconds or more, alternative solutions to the problem of voltage drop during charging have been proposed. One such solution is to provide two batteries, one for charging the output capacitor and a separate battery for powering the control circuit. Such was the case, for example, in the Model 7210 ZSX implantable cardioverter, manufactured for clinical investigation by Medtronic, Inc., Minneapolis, Minn. One problem with this approach, is that the relative amounts of energy required in a device for the control and charging/output circuitry will vary from patient to patient. The capacity of the battery to power the control circuit can only be optimized with regard to one patient profile. For all other patients, one battery will deplete before the other battery, leaving wasted energy in the device. Thus, for a given required device longevity, the overall battery capacity needed to power the control circuit and the charging/output circuit separately with two power sources can be greater than the capacity needed to power them together with one power source.
As a practical matter, the battery chemistries used in implantable defibrillators do not provide a single cell voltage levels which will simultaneously allow for acceptable charging times, provide adequate voltage levels during capacitor charging and provide a reasonable device longevity. Thus, commercially marketed implantable defibrillators employ multiple cell batteries to provide sufficiently high voltages. However, this approach still provides less than optimal device longevity, for the reasons discussed above. Further, if possible, a single cell battery would be preferable to a multiple cell battery of the same capacity.